I bought a $10 knife from Walmart for both my PSK, as well as my Altoids kit. I can't find it on the walmart website, but I think this might be it:

http://www.kershawknives.com/productdetails.php?id=138&brand=kershaw

Lockback, small folder, very thin. Probably OK for a few days use. I know that Buck also sells a 2" lockback for the keychain too, probably aruond $25.